1. Field of the Invention
Polylactide and polyglycolide are biodegradable polymers which are manufactured commercially. The monomer used is lactide or glycolide which are cyclic dimers of lactic acid or glycolic acid and which are prepared from lactic acid or glycolic acid.
2. Prior Art
U.S. Pat. No. 5,023,349 discloses a process for the rapid conversion of oligomers of alpha-hydroxycarboxylic acids, esters or salts thereof to cyclic dimer esters in high yields by rapidly passing an inert gas through the reaction mixtures at a temperature such that the cyclic dimer is rapidly removed from the reaction mixture as it is formed. The process uses 0.1 to 1.5% and preferably 0.3 to 0.9% stannous octoate catalyst.
U.S. Pat. No. 5,053,522 discloses heating lactic acid, glycolic acid, or polylactic or polyglycolic acid of molecular weight 400 to 2,000, preferably 500 to 800, at 130.degree. to 230.degree. C., preferably 180.degree. to 200.degree. C., in the presence of 0.05 to 1.0 weight percent, preferably 0.1 to 0.8 weight percent of a tin catalyst to form lactide. The lactide formed is distilled off and additional polylactic acid is fed in continuously or batchwise.
The higher molecular weight oligomer produced by the presently used processes requires treatment to lower its molecular weight before it can be recycled to the lactide or glycolide preparation step. The need for this treatment to lower the molecular weight of the oligomer is a necessary part of the conventional lactide or glycolide production.
This high molecular weight material presents two problems. First, if the molecular weight is allowed to go too high, the viscosity increases to the point where the material becomes intractable. Second, to prevent yield loss, this high molecular weight material must be re-hydrolyzed and recycled.
Dilution is a technique which can be used to control molecular weight of the oligomer remaining in the reaction mixture after cyclic dimer production. The reactions which produce cyclic dimer are uni-molecular whereas the dehydration reactions are bi-molecular. Thus the rate of dimer production is linearly proportional to oligomer concentration while dehydration is proportional to the square of the oligomer concentration. Reducing the concentration of monomer in the reactor by one half would lower the dimer production rate by one half but reduce the rate of molecular weight increase of oligomer by one half squared or one quarter. At present this is the only method known to limit the increase in molecular weight of the oligomer remaining in the reactor and this method has only marginal success. Dilution however reduces productivity and thus increases costs.